Thanks, this suggestion was actually already made by the JWT maintainer (ambs) in github:

my $jwt_plugin = $app->with_plugin('Dancer2::Plugin::JWT');
my $jwt = $jwt_plugin->jwt();
[download]

and, as you suggest, this:

my $jwt_plugin = $app->find_plugin('Dancer2::Plugin::JWT');
my $jwt = $jwt_plugin->jwt();
[download]

seems to work also.